1. Field of the Invention
The present invention relates generally to an imaging apparatus and an imaging method.
2. Description of the Related Art
Conventionally, a digital still camera, a camera-mounted cellular phone, a digital video camera, and so forth provide an Auto Focus (AF) function for automatically adjusting focus. The AF can be divided roughly into two schemes: a phase difference detection scheme for optically measuring a distance to an object by using a range-finding sensor and a contrast detection scheme for detecting a position at which a contrast of an image has a peak value as a focusing position. The contrast detection scheme will be described below.
The contrast detection scheme extracts a high-frequency component of an input image signal and controls a focus lens so as to maximize a contrast value (which will hereinafter be referred to as the AF evaluation value) calculated based on the high-frequency component, thereby adjusting focus. FIG. 6 is a diagram illustrating the contrast detection scheme.
While a High-Pass Filter (HPF) is used to extract the high-frequency component of the image herein, an HPF in a horizontal direction with respect to an image is generally used. For example, by applying a horizontal HPF to an image of vertical stripes as shown in FIG. 7A, a high-frequency component as shown in FIG. 7B can be extracted. However, for an image of horizontal stripes as shown in FIG. 8A, a high-frequency component cannot be extracted as shown in FIG. 8B. That is, AF cannot be implemented.
To solve this problem, a technique for extracting a high-frequency component in a vertical direction has been proposed. For example, a technique disclosed in Japanese Patent Application Publication No. 2005-326621 includes a 2nd order filter to extract high-frequency components in horizontal and vertical directions.
A technique disclosed in Japanese Patent Publication No. 2007-94236 rotates an input image by 90° and applies a filter in a horizontal direction, thus obtaining the same effect as when the filter is applied in a vertical direction. In the technique disclosed therein, a vertical-direction 1st order filter is provided in addition to a horizontal-direction 1st order filter. Thus, by including the vertical-direction filter as well as the horizontal-direction filter, a high-frequency component can be extracted with respect to an image of horizontal stripes as shown in FIG. 8C, and the AF can be performed on a scene, such as a horizontal line, with high precision.
However, the conventional technique has some problems as will be discussed below. For example, a technique disclosed in Japanese Patent Publication No. 1997-107495 uses a 2nd order filter in which the increase of the number of taps adjusting a coefficient of the 2nd filter for improving focusing precision results in the increase of a memory size.
The technique disclosed in Japanese Patent Application Publication No. 2005-326621 requires a memory region for storing a 90° rotated image and increases the memory cost.
Patent Document 3 does not disclose a detailed structure of an HPF. In particular, a method for implementing a vertical-direction HPF with low cost. As a result, to obtain steep cut-off characteristics with a generally used Finite Impulse Response (FIR)-type filter, the number of taps increases and thus the memory size also increases.